Freon-polyurethane foam heat-insulator utilizes low heat conductivity of Freon and has been used generally as an excellent heat-insulator. Since the use of Freon has been restricted in recent years, a heat-insulator capable of exhibiting an excellent heat-insulating performance without using Freon has been demanded.
The preparation of a heat-insulator has recently been provided, prepared by incorporating fine inorganic particles such as silica powder as a heat-insulating core material into a flexible container (bag) having gas barrier property, then evacuating the inside of the container to a reduced pressure state, thereby closely adhering the fine inorganic particles to the container, and sealing the container.
For example, in the case where such a vacuum heat-insulator is used in a refrigerator or device for cool storage or for cold preserving storage, it is preferred that the thermal conductivity at 20.degree. C. of the vacuum heat-insulator be 0.005 to 0.008 Kcal/m.multidot.hr.multidot..degree.C. When the thermal conductivity is more than 0.008 Kcal/m.multidot.hr.multidot..degree.C., excess electric power and current is required for maintaining the cold storage temperature. If a vacuum heat-insulator having a larger thermal conductivity is used, a much more thicker heat-insulator must necessarily be used, undesirably reducing the size of the storage area.
However, the vacuum heat-insulator described above involves a problem of emitting powdery dusts upon discarding to bring about an environmental problem. The present invention has been accomplished in view of the foregoing situations and an object thereof is to provide a novel vacuum heat-insulator bringing about no environmental problem caused by dusting upon discarding and capable of exhibiting an excellent heat-insulating performance without using Froen, as well as a method of preparing the same .